Gas compressors such as those used in air conditioning and like systems use oil or alternatives as a lubricant for the compressor bearings. Because lubricating oils have an affinity with and absorb the refrigerants in which they operate, they should ideally be kept at an elevated temperature even when the compressor is not operating to prevent the refrigerant condensing in the oil. Such condensed refrigerant causes oil to foam on initial starting of a compressor, ultimately leading to compressor failure.
Further, up until now it has been necessary to design the refrigeration circuit of an air conditioning system to ensure that any oil which travels through the system can be returned to the compressor. Because it is difficult to restrict or prevent the oil travelling through the entire refrigeration system, oil traps need to be placed and oil return has to be taken into account when the system is designed. This causes restrictions such as the need to limit equipment location, the length of pipe run, the size of the refrigerant piping and the nature of the equipment used in the system. Because of the need to take these factors into consideration, the efficiency of a system and the operating ability of the system, such as the ability to unload can be compromised.
Most refrigeration and air conditioning systems currently use a refrigerant R12 or a singular refrigerant which is a CFC or HCFC refrigerant which is potentially damaging to the environment. Other refrigerants in use include R22, which is currently approved for use under the Montreal Protocol on the ozone layer until 2030 A.D. However, use of this refrigerant must be in progressively reducing volumes and the only CFC-free commercial refrigerant currently endorsed without reservation by the Montreal Protocol and by the International Heating, Ventilation and Air Conditioning Industry (HVAC) is the refrigerant known as R134A. This refrigerant, however, is commercially unsuitable as a direct replacement for the CFC refrigerants in existing hematic or semi-hematic machines because the chemical structure of R134A results in a performance loss of up to about 30%. Further, the refrigerant R134A is basically unsuitable for use with existing compressors because the refrigerant is chemically incompatible with lubricants now available for the mechanical bearings and other rotating or reciprocating parts of the compressors.
Another difficulty with current air conditioning systems is that, traditionally, small to medium refrigeration systems of between 1 and 150 kilowatts use reciprocating, rotary or scroll compressors which are relatively cheap to produce but are relatively inefficient. Screw compressors become more efficient at sizes between 150 and 1,000 kilowatts although most systems over 500 kilowatts use centrifugal compressors. These are more efficient than screw compressors, but are conventionally far more costly to produce and maintain.
The efficiencies of the smaller equipment, below 180 kilowatts, is restricted by the available technology in the reciprocating, rotary, scroll and screw compressors. While centrifugal machines can offer a higher efficiency in the lower capacity range, limitations on high rotational speed drives, and the cost thereof, inhibits their use.